The present invention relates to an apparatus and methods for dispensing liquids in accurate quantities, in particular fluids with high viscosity and fluids used in fabrication processes, such as semiconductor device fabrication processes, in which process fluid waste and contamination are of particular concern.
Many processes require accurate control over the amount and/or rate at which a fluid is dispensed by pumping apparatus. Both the rate and amount of processing fluid applied to, for example, a semiconductor wafer during fabrication of integrated circuits are very accurately controlled to ensure that the processing liquid is applied uniformly, and to avoid waste and unnecessary consumption. Many of the chemicals used in the semiconductor industry are toxic and very expensive. Accurate dispensing thus avoids toxic waste handling and reduces cost of fabrication. Contamination of process fluid in the form of air bubbles or particles or other external contamination must also be carefully controlled in many processes. Contamination in semiconductor device fabrication processes, for example, lowers yields and results in lost process fluid and production time.
For example, the manufacture of multi-chip modules (MCM), high-density interconnect (HDI) components and other semiconductor materials requires the application of a thin layer of polyimide material as an inner layer dielectric. The polyimide material must be applied with exacting precision because the required thicknesses of the polyimide film may be as small as 100 microns and the final thickness of the polyimide film must be uniform and not normally vary more than 2% across the substrate or wafer. In addition to the unique mechanical and electrical properties that make polyimides ideally suited for use in the manufacture of semiconductors, polyimides also have physical properties that make it difficult to pump or supply the polyimides in exacting amounts. Specifically, polyimides are viscous. Many polyimides used in the manufacture of semiconductors have viscosities in excess of 400 poise. Fluids with viscosities this high are difficult to pump and difficult to filter. It is not uncommon for polyimide fluids to cost in excess of $15,000 per gallon. Therefore, it is important that pump systems used to dispense the polyimide fluids dispense the exact amounts, without waste.
Fluid dispense systems in the prior art normally use positive displacement pumps to provide accurate metering of fluid. One type of positive displacement pump sometimes used in prior art is a bellows-type pump, an example of which is disclosed in U.S. Pat. No. 4,483,665. In a typical bellows pump, fluid to be pumped enters a hollow tubular bellows through a one-way check valve. Usually, the discharge end of the bellows is constrained from movement, while the other end is connected to a reciprocating mechanical member that selectively works the bellows for longitudinal expansion and contraction. When contracted, fluid is expelled or pumped from the bellows under pressure. One problem with a bellows pump is that, at high pumping pressures, considerable internal pressure is exerted on the bellows which, together with flexing during expansion and contraction, can result in fatigue and rupture of the bellows. Furthermore, the bellows will flex under pressure, causing a loss in the accuracy. To overcome this problem, fluid is pumped into a chamber surrounding the bellows to balance at least partially the pressure of the process fluid within the bellows. Another problem with bellows is that the pleats or convolutions in the bellows make it difficult to purge completely air or chemicals from the bellows. Air remaining in the bellows can create undesirable air bubbles.
A diaphragm-type positive displacement pump overcomes some of the problems associated with a bellows type of pump. A diaphragm pump has a diaphragm that divides a pumping chamber into in two sections. A working fluid is pumped into and out of one section of the chamber to cause the diaphragm to move back and forth, thereby forcing process fluid to be drawn into and pushed out of the other half of the chamber. If the change in the volume of the working fluid within the chamber is accurately known, the volume of the process fluid within the chamber can also be known accurately, thus allowing for accurate metering. Diaphragm pumps are therefore often actuated by incompressible hydraulic fluid to achieve very accurate control over movement of the diaphragm. Examples of diaphragm pumps are disclosed in U.S. Pat. Nos. 4,950,134, 5,167,837, 5,490,765, 5,516,429, 5,527,161, 5,762,795, and 5,772,899.
However, should a hydraulically actuated diaphragm fail, such as by developing a hole, hydraulic fluid may be forced into process fluid. This contamination then flows downstream, for example into other systems or onto, for example, semiconductor substrates that are then, in turn, processed, thus contaminating other systems down the production line. Furthermore, when servicing these systems hydraulic fluid may be tracked through a xe2x80x9cclean roomxe2x80x9d environment on tools, gloves and other equipment, potentially contaminating the clean room. To avoid possible contamination by hydraulic fluid, the diaphragm could be pneumatically actuated. However, the compressibility of air makes accurate control of the dispense volume more difficult.
Another type of well known positive displacement pump is a rolling membrane pump. A rolling membrane pump includes a reciprocating piston that displaces fluid within a pumping chamber. Unlike piston-type pumps that have a moving seal between the piston and the pumping chamber walls, a flexible membrane is attached to the piston and to the side walls of the chamber to prevent fluid from escaping between the walls and the piston. As the piston moves, the membrane rolls up and down the side of the pump. However, the membrane flexes stretches under high pressures. Many of the process fluids that must be dispensed in semiconductor fabrication processes are highly viscous, and must be pumped at very high pressures. Presumably, for this reason it does not appear to have been used in prior art systems for accurately dispensing small quantities of liquid, particularly those in fabrication processes of semiconductor devices.
The invention provides for an improved precision fluid dispensing apparatus and method that solves on or more of the problems found in the prior art. More particularly, the invention avoids use of hydraulic fluid as a working medium to pump process fluids, thereby reducing risk of contamination to the process fluid and production environment, and overcomes problems associated with other types of positive displacement pumps to provide for accurate fluid dispensing.
According to one aspect of an exemplary embodiment of the invention, the problems with using a rolling membrane pump to meter accurately process fluid are overcome. The change in volume in a pumping chamber of the rolling membrane pump due to stretching is predicted to an acceptable degree as a function of pressure within the pumping chamber. The pressure of the process fluid within the chamber is monitored throughout a displacement stroke, and the distance of the displacement stroke necessary to deliver a preselected quantity of process fluid updated throughout the stroke to take into account and correct for the flexing and stretching of the membrane. The risk of contamination of process fluid is substantially reduced by not using hydraulic fluid to work a diaphragm for pumping process fluids, relying instead on a solid mechanical actuator of a membrane. Furthermore, unlike prior art bellows pumps, a rolling membrane pump has no convolutions and thus can be easily purged and cleaned.
According to another aspect of a preferred embodiment of the invention, a high precision dispensing system is made easier to maintain by use of a rolling membrane pump head that is coupled to a mechanical actuator powered by an electric motor that may be easily disconnected. Thus, the entire fluid path, consisting of the pumping chamber, chamber body, rolling membrane, a displacing mechanism, such as a piston, valves and fluid connections may be easily removed from a clean room environment for servicing without disturbing the mechanical actuator and controller. A second, clean pump head may thus be installed allowing the system to be returned to operation very quickly. The pump head may also be easily cleaned and reinstalled. The internal shape of the rolling membrane allows for it to be flushed rapidly. Thus, costly down time in a production facility can be avoided. Similarly, separation of the pump head from the drive mechanism allows the drive mechanism to be easily serviced and replaced, if necessary. Since the process fluid path would not be disturbed, there would be no fluid loss or purging required to remove air from the process fluid flow path.
Another advantage to the invention is that it is capable of being used with a wide range of process fluids, having very low viscosity (on the order of 1 to 2 centipoise) to very high viscosity (over 300 poise). Examples of such process fluids include, but are not limited to, solvents, resists, spin on glass (SOG), polyimides, low dielectric and many other chemistries used in semiconductor device fabrication processes. Although well suited for semiconductor device processing applications, the invention may be used in other applications.
In the preferred embodiment, the method comprises calculating an amount by which to change a dispense based at least in part on a predicted membrane flex if a particular dispense is other than a first dispense, wherein said predicted membrane flex is based at least in part on a maximum pump chamber pressure during the first dispense; calculating an amount by which to change a dispense based at least in part on a shape of the membrane if a particular dispense is a first dispense; moving a piston in the pumping system based at least in part on the calculated amount; opening an output valve of the pumping system; monitoring the pump chamber pressure to detect a sudden decrease in said pump chamber pressure to signal a mechanical failure in the pumping system; and determining a maximum pressure in the pump chamber during the movement of the piston.
Following is a detailed description of an exemplary embodiment of the invention, made in reference to the appended drawings.